Abstract Smoking cessation interventions (pharmacological and behavioral) and policy actions have reduced overall smoking rates. However, there remain significant populations for whom success rates are lower than predicted, most notably smokers who use other substances and those with psychiatric disorders. Cannabis, the most prevalent illicit drug in the U.S. and among tobacco users, may increase risk for relapse to cigarette smoking. Stress is one of the most commonly reported precipitants of smoking and relapse for both tobacco and cannabis users. Systems involved in the stress response, including the hypothalamic-pituitary-adrenocortical (HPA), endocannabinoid, and sympathetic systems, play a critical role in regulating mood and the reinforcing effects of drug use. Research from our current and previous funding periods has shown that cigarette smokers exhibit: 1) enhanced basal HPA activity; 2) blunted cortisol response to multiple stressors; 3) disrupted opioid regulation of the stress response; 4) association of attenuated stress response and early smoking relapse; and 5) sex differences in patterns of predictors of relapse-- men exhibit consistent hormonal prediction, while women show consistent withdrawal symptoms predictive of smoking relapse. It is not known whether the altered stress response observed in smokers is further exacerbated by cannabis co-use. How these systems are modified by withdrawal and influence risk for relapse is also unknown. Research to elucidate sex differences in these factors is particularly lacking. We propose to elucidate stress-related psychobiological mechanisms associated with concurrent cannabis use that impede successful smoking cessation and which, when identified, can be used as metrics to identify and tailor interventions. We will therefore conduct this research to: 1) Define the impact of concurrent cannabis use on the psychobiological mechanisms of the stress response in smokers who are interested in quitting but still smoking ad libitum, 2) determine the impact of abstinence from cannabis use, in conjunction with abstinence from cigarette smoking, on stress response in smokers who are interested in tobacco cessation, 3) examine the extent to which stress response during ad libitum smoking and during abstinence from tobacco, or from tobacco and cannabis, predict affective disturbance and smoking relapse over three months, and 4) identify sex differences in diurnal patterns and the stress response during ad libitum smoking and withdrawal in smokers and co-users, and establish the extent to which these differences predict relapse. The proposed work will build on our previous research to set the stage for interventions that involve a personalized medicine approach to predicting smoking relapse. This will enable the development and testing of targeted, sex-specific, just-in-time interventions to address the impact of stress and co-use of cannabis on smoking cessation.